1. Field of the Invention
The present invention relates to an over-current protection device and a method for manufacturing the same.
2. Description of the Related Art
When switching temperature, conductive composite with a positive temperature coefficient (PTC) will be converted from a low resistance state to a high resistance state. When an over-current protection device made of such conductive composite connects in series with an external load on the electric circuit, under normal operating condition, the over-current protection device demonstrates low resistance. However, when high current passes through the over-current protection device or when the device is heated to high temperature, the resistance immediately rises. The change in resistance effectively limits the current passing through the over-current protection device, and therefore protects the electronic devices in the electric circuit.
A general PTC conductive composite comprises one or more polyolefin polymers and conductive filler, among which the polymer can be polyethylene, polypropylene, and/or polymethylmethacrylate; the conductive filler can typically be carbon black, metal particulates (e.g., nickel, gold, silver, etc.), or oxygen-free ceramic powder (e.g., titanium carbide, tungsten carbide, or their eutectic materials). However, polyolefines has a crystalline melting temperature of less than 130 degrees Celsius, causing devices made of polyolefines to behave abnormally when temperature changes drastically.
U.S. Pat. Nos. 4,859,836 and 5,317,061 disclose a conductive composite, which includes tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene and perfluoro (propylvinyl ether) copolymer (PFA), irradiated polytetrafluoroethylene (PTFE), and carbon black. The high crystalline melting temperature (270 to 340 degrees Celsius) of FEP and PFA makes the aforementioned conductive composite difficult to manufacture. In addition, when processed at high temperature, the preceding conductive materials are inclined to pyrolyse and produce corrosive gases. Moreover, the high crystalline melting temperatures of FEP, PFA, and PTFE lead to excessive high temperature when the device is activated, and further melts the tin solder at the welded point. Consequently, damage at the joint or distortion of the plastic fixture may occur.
U.S. Pat. No. 5,451,919 discloses another conductive composite, which comprises polyvinylidene fluoride (PVDF), ethylene/tetrafluoroethylene (ETFE), and carbon black. In some embodiments, photo-crosslinking agent-triallylisocyanurate (TAIC) and calcium carbonate (CaCO3) are added to conductive composite. Under irradiation, TAIC facilitates the polymer cross-link reaction and improves the stability of product size and operating temperature. Based on experimental results, adding ETFE enhances the stability of over-current protection devices. However, employing ETFE material in conductive composite manufacturing requires high processing temperatures (at least 260 degrees Celsius). This high temperature process pyrolyses a small quantity of PVDF, and generates corrosive gases such as hydrofluoric acid. Although adding alkaline fillers such as CaCO3 promotes neutralization, this method increases manufacturing cost because processing equipment used to manufacture the aforementioned conductive composite requires special alloy material.
In addition to the above-mentioned disadvantages, extra care must be taken concerning the problems generated from the installation of over-current protection devices in harsh environments. For instance, an over-current protection device installed under a car engine hood not only will be affected by high temperature caused by engine operation, but also needs to withstand drastic climate changes such as cold, heat, dryness and humidity that occur outside of the car. Conventional over-current protection devices could operate only under well-controlled environments. Therefore, an over-current protection device that can function stably at high temperature and in drastically changing climates is under expectation.